This invention relates to underwater communications systems. In particular, this invention relates to two-way underwater communications between a buoy and submerged craft via magneto-inductive signals propagated in the extremely low frequency (ELF) to very low frequency (VLF) range and RF and laser emission links to other stations.
Current methods used to communicate between submarines and surface command locations are limited. Some submarines surface and transmit RF messages to orbiting satellites which in turn relay communications to command centers. Another communication method uses sonar signals to communicate short messages to a buoy that in turn relays the message to satellite using an RF link. Another communication method releases a cable tethered buoy to communicate RF signals from the submarine, through the cable, and to the buoy which are then transmitted to a satellite. Each of these methods has limitations and disadvantages.
Two way communications are possible when using a buoy tethered to a cable. However, this requires the submarine to remain close to the surface and fairly stationary to have reliable communications. Furthermore, this communication method makes the submarine noisy at the water's surface where it is susceptible to passive sonar detection. Because the submarine has to move close to the surface in order to tether a surfaced buoy, the submarine also is vulnerable to visual, RF, and LASER detection by satellite and other military surveillance craft. Additionally, since the submarine is continually tethered to the buoy, monitored transmissions from the buoy can be triangulated and, consequently, the location of the submarine is revealed.
Submarines that surface as they are communicating with one or more satellites, are vulnerable to many types of detection and surveillance. Submarines using sonar to communicate signals to untethered relay buoys may be detected by passive means. In addition, the signal clarity and baud rate of sonar transmissions also are limited by environmental factors, such as sea state, wave height, reverberation, and particulates, as well as water salinity, temperature and thermoclines. Use of sonar may shorten the rate and range of communication between buoys and submarines.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a buoy and system using magneto-inductive signals in the ELF to VLF range for two-way communications between a submarine and the buoy that is linked by RF or laser emissions to extend these communications to other stations and does not compromise the submarine's location.